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CHE116_Lecture6_2011

CHE116_Lecture6_2011 - Chemistry Chemistry116 Lecture6...

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Chemistry 116 Lecture 6 Topics: Using Equilibrium Constants Chapter 15.4 15.6
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The Concept of Equilibrium Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate. Key concept is N 2 O 4 ( g ) 2 NO 2 ( g ) that this reaction is reversible . clear brown
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The Equilibrium Constant The ratio of the rate constants is a constant at The ratio of the rate constants is a constant at that temperature, and the expression becomes: K eq = k f k [NO 2 ] 2 [N O ] = r [N 2 O 4 ] This is known as the equilibrium constant .
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The Equilibrium Constant Consider the generalized reaction: b d The equilibrium expression for this reaction aA + bB cC + dD (from the law of mass action ) would be: K c = [C] c [D] d [A] a [B] b [A] [B] The equilibrium constant depends only on the stoichiometry of the reaction, not the mechanism!
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What Does the Value of K Mean? If K >>1 , the reaction is product favored ; product predominates at equilibrium. • If K <<1 , the reaction is reactant favored ; reactant favored reactant predominates at equilibrium at equilibrium.
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Heterogeneous Equilibria The equilibrium examples we have talked about f b d h ilib i so far are based on homogeneous equilibria. Homogeneous equilibrium means that all of the reactants and products are in the same physical state or phase. Heterogeneous equilibrium considers the equilibrium between components in different phases ( e.g. a solid placed in a liquid).
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Heterogeneous Equilibria The concentrations of pure solids and liquids are constant. Solid and liquid concentrations can be obtained by multiplying the density of the substance by its molar mass. Th t t t t t t t These are constants at constant temperature . For example, doubling the mass of a pure solid would double its volume and result in an unchanged density (density concentration).
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Heterogeneous Equilibria Therefore, the concentrations of solids and liquids d i h ilib i i do not appear in the equilibrium expression.
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